In its latest escalation of rhetoric over the disputed islands, Argentina said it had written to Falkland Oil & Gas (FOGL), Borders & Southern, Rockhopper, Desire Petroleum and Argos Resources on April 17, to "notify them of their illicit actions and their consequences".

"In case of failure to offer a response and once the deadline expires, administrative sanctions will be imposed to each company within the framework of an Energy Secretariat resolution which deems these activities illegal," it said. "The Argentine government will also press criminal and civil charges."

It gave no details of the form in which it intended to pursue such action, however. None of the explorers has any assets in Argentina and the British Foreign Office has said it is "deeply sceptical" that Argentina could pursue penalties against such companies in foreign courts.

Argentina said in March it planned to sue the explorers, but this is thought to be the first time this year it has written to them and the first time it has set a deadline. Legal letters have been sent to the companies in previous years, and in March were sent to banks who advised them, but no legal action has resulted.

Earlier this month, FOGL chief executive Tim Bushell shrugged off the "political noise" from Argentina: "Our legal advice is there are no grounds under any international law for pursuing their claims," he said. The companies either declined to comment or could not be reached for response on Thursday night but it is not thought any of them intends to reply to the letters.

Meanwhile, the head of Repsol in Argentina sold more than half his shares in the Spanish oil group ahead of Buenos Aires' decision to expropriate its stake in YPF, which sent Repsol's share price tumbling.

Antonio Gomis sold 9,424 shares in November at an average of more than €22 each, days after YPF discovered the huge Vaca Muerta shale gas field and also following Argentine government opposition to a YPF dividend payout. Shares in Repsol began falling in January as Argentine rhetoric turned against Repsol, and traded at €14.46 on Thursday.

Repsol said Mr Gomis acted lawfully and had no idea of the expropriation in November as Argentina was still broadly supportive. Mr Gomis sold the shares for private reasons, it said, and on the only day that Spanish law allowed in that quarter.

Argentina's Economy Minister Hernan Lorenzino said on Thursday night that his country must seize control of YPF to boost production of the oil and natural gas needed to fuel economic growth.

"This decision is about energy self-sufficiency, which is very important for our country," said Mr Lorenzino, a former finance secretary who took office in December.

Mr Lorenzino, 40, said the expropriation push also responded to Repsol's "failure to fulfill its obligation to increase... reserves and production in a way that keeps up with growth and sustains Argentina's economic activity."